Flux Growth, Crystal Structure, and Chemical Bonding of YbPdGe, an AlB Superstructure within the Rare-Earth Series.

The complete structure revision of the PdGe ( = rare-earth metal) series revealed that YbPdGe is the only AlB ordered superstructure. Good-quality single crystals of this compound were successfully grown from molten indium flux, enabling accurate single-crystal investigations. YbPdGe crystallizes with the CeCoSi-type structure in the hexagonal space group 6/ (no. 191) with lattice parameters = 8.468(1) Å and = 4.0747(7) Å. This structure is a four-order derivative of AlB, composed of planar [PdGe] honeycomb layers spaced by Yb species, located at the center of Ge and GePd hexagons. A superconducting transition is observed below the critical temperature of 4 K. A divalent state of Yb is deduced from magnetic susceptibility measurements below room temperature, which indicate an almost nonmagnetic behavior. A charge transfer from Yb to Pd and Ge was evidenced by the Quantum Theory of Atoms in Molecules (QTAIM) effective charges; polar four-atomic Ge-Pd/Yb and two-atomic Pd-Yb bonds were observed from the ELI-D (electron localizability indicator), partial ELI-D, and ELI-D/QTAIM intersections. The bonding interactions between Ge atoms within regular Ge hexagons are found to be intermediate between single bonds, as in elemental Ge, and higher-order bonds in the hypothetic GeH and Ge aromatic molecules.